The present invention relates to an apparatus and method for rapidly locating and determining the extent of damage in bonded and solid laminate structures, particularly fiber-reinforced resin matrix composites.
Bonded metal and composite structures and monolithic metal and composite structures are widely used in the manufacture of aircraft. The percentage of composite materials being used on aircraft automobiles, boats, and in building construction materials continues to rise. Use increases the need for Non-Destructive Inspection (NDI) methods that can effectively identify composite degradation and damage. Delamination caused by impact can significantly reduce the strength or stiffness of a composite without being visible to the naked eye. A simple, cost effective inspection method to detect such defects is needed. Methods such as echo ultrasonics, shearography, and thermography are costly to implement. These methods also often require highly trained operators to implement, and specialized, bulky equipment.
A simple composite NDI technique is the coin-tap or tap hammer method. In this method, the inspector will use an ordinary coin, such as a U.S. quarter, or a small hammer to tap along the structure and listen for a change in the sound. This method works because of the difference in the sound produced by a bad (e.g., delaminated) region versus a good region. A good region tends to ring while a bad region will sound dead. The mechanics of the tap testing method have been studied in detail by P. Cawley and R. D. Adams, who published, for example: Sound and Vibration, 122,(2), 299 (1988); Mat""l Eval., 47, 558 (1989), Brit. J. NDT, [32(9) 454 (1990); Int""l Conf. Structural Adhesives in Eng""g, C19, 139 (1986), or U.S. Pat. No. 5,589,635, which we incorporate by reference. Theoretical and experimental results from the Cawley and Adams work demonstrate the viability of using changes in features of the force-time curve of the impact as an indication of the local integrity of the structure. Damage, such as a disbond, results in a local decrease in the stiffness of the structure, which changes the force-time curve that a tap near the damage produces. The difference in the audible sound tells one that the force-time curves differ, but the tap method is difficult to implement in a noisy environment. The amplitude, duration, and frequency response of the impact curve are all affected by the local stiffness, and under various conditions can be flagged as indicators of defects and damage.
Although it is cheap and simple, the tap method has several drawbacks. It depends upon the inspector""s hearing, experience, and interpretation. The results are subject to interference from workplace noise. Workplace noise is especially a problem in an on-aircraft, flightline environment. The traditional tap method is unable to provide quantitative data. Quantitative data would provide a clearer indication for the flightline inspector to make a fly or no-fly determination. Tapping on the surface with a coin or tap hammer and listening to the tone of the part is adequate for limited detection. It is subjective and is often imprecise.
Approaches to overcome these drawbacks continue to be relatively expensive and are of limited practical use for on-aircraft inspection.
The damage detection device makes it easy to locate damage to both solid and bonded surfaces, such as an aircraft wing skin, a car door, a boat hull, or the like. The device does not change the operator""s ability to listen for audible tonal changes, but it provides a quantitative reference number in microseconds that represents the state of stiffness of the surface. This time measurement can be correlated to a level of damage. The ability to locate damage using an instrument that provides a reference number will increase the confidence of the technician and both the accuracy and repeatability of the inspection.
We have developed a simple low cost instrumented tap hammer that provides a quantitative measure of the hammer/composite impulse time. We have correlated the time (duration) measured with the amount of damage (delaminations) in the structure. The instrumented tap hammer of the present invention supplements the tonal discrimination of the operator with a numeric readout that can readily be related to local part quality. The effect of background noise and operator differences on the inspection results can be eliminated. An increased sensitivity is also shown over the audible tap test method.
In a preferred embodiment our damage detection device includes a tap hammer with an acceleration sensor mounted in the head of the hammer. A wire connects the sensor to an analysis circuit that converts an impact signal from the hammer into a readout value. The circuit measures a time in microseconds during which the impact signal remains above a predetermined threshold. The readout value is then shown on a display.
A second, miniaturized embodiment of the device includes an acceleration sensor, a circuit, and a display, in the shape of or mounted on a slug or a coin. The inspector taps the structure with the acceleration sensor like the coin tap method to produce an impact signal which the circuit converts to a readout value, which is displayed. The readout can be xe2x80x9cpass/fail,xe2x80x9d the impulse time, or some other information, as desired.
The damage detection device, including a tap hammer, is inexpensive and easy to use. It provides quantitative results without relying on elaborate or expensive circuitry. The slug or coin device, is small, easy to use, and extremely portable. It is used just like the current coin tapping process, but with more positive results. The readout provides a quantitative value to assess the condition, even if there is significant background noise.
The damage detection device addresses the genuine need for a low cost instrumented non-destructive evaluation (NDE) method for factory or field testing of composite commercial and military aircraft. The results of testing demonstrate the benefit of a quantitative indication of the local stiffness (as measured by the impact pulse width). The use of the damage detection device provides a clear improvement over the traditional coin tap method by reducing its subjectivity, increasing its sensitivity, and quantifying the actual impact response.
The skin thickness is an important consideration in whether a composite can be effectively inspected. As the skin thickness increases, the difference between the response of xe2x80x9cgoodxe2x80x9d and disbonded skin will get smaller. The effective range for the damage detection device was 1-7 plies on fiberglass/epoxy facesheets of the sandwich panels tested. The range will depend upon the material and type of structure, as well as the defect type, size, and depth. We are able to detect delaminations in 6 mm thick graphite/epoxy composite laminates.